"Park trailer" or "park model trailer" means a travel trailer designed to be used with temporary connections to utilities necessary for operation of installed fixtures and appliances. The trailer's gross area shall not exceed four hundred square feet when in setup mode. "Park trailer" excludes a mobile home. RCW 46.04.622

When a 'park model' has substantially lost its identity as a mobile unit by virtue of being permanently sited in location and placed on a foundation of either posts or blocks with connections with sewer, water, or other utilities for the operation of installed fixtures and appliances the "park model" meets the definition of "real property". RCW 82.50.530

Yes. An excise tax is imposed (or due) on the sale of real property. Revised Code of Washington (RCW) 82.45.060 Tax on sale of property.

(1) "Real estate" or "real property" means any interest, estate, or beneficial interest in land or anything affixed to land, including ownership interest or beneficial interest in any entity which itself owns land or anything affixed to land. The term includes used mobile homes, used park model trailers. RCW 82.45.032